A=[1  2 -3 1; -3 -8 7 1];
A(2,:)=A(2,:)-A(2,1)/A(1,1)*A(1,:);
A(2,:)=A(2,:)/A(2,2);
A(1,:)=A(1,:)-A(1,2)/A(2,2)*A(2,:);
A
